Figure 11.5: Sponges have tube-like bodies with many pores. ( 11 )Sponges are relatively primitive animals and do not have brains, stomachs, or other organs.
In fact, sponges do not even have true tissues. Instead, their bodies are made up of special-
ized cells that each has specific functions. For example, the collar cells are flagellated and
encourage water movement, while other types of cells regulate the water flow by increasing
or decreasing the size of the pores.
Cnidarians
The cnidarians, in the phylumCnidaria, include organisms such as the jellyfish (Figure
11.6) and sea anemones (Figures11.7and11.8) that are found in shallow ocean water.
You might recognize that these animals can give you a painful sting if you step on them.
That’s because cnidarians have stinging cells known asnematocysts. When touched, the
nematocysts unleash long, hollow threads that are intended to trap prey, and sometimes
toxins are also injected through these threads to paralyze the prey.
The body plan of cnidarians is unique because these organisms are radially symmetrical,
meaning that they have a circular body plan so that any cut through the center of the
animal leaves two equal halves. The cnidarians have two basic body forms, polyp and
medusa. Thepolypis a cup-shaped body with the mouth directed upward, such as a sea
anemone (Figure11.8). Themedusais a bell-shaped body with the mouth and tentacles
directed downward, such as a jellyfish (Figure11.7).
Unlike the sponges, the cnidarians are made up of true tissues. The inner tissue layer secretes
digestive enzymes into thegastrovascular cavity,a large cavity that has both digestive